you would have to trade him from another game
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you c'ant capture entei in Pokemon ruby do you understand
Entei, Suicune, and Raikou are not available in Pokemon Emerald, Ruby or Sapphire. They can be brought into the game by trading.
use a articuno and use his attack ice heal and ice thrower
You cannot get the Legendary Dogs (or Beasts such as Raikou, Suicune and Entei) in Ruby without either trading for them or cheating for them because they are not available in the wild in Ruby in any way, shape or form. But you can catch it in Pokemon Ruby Destiny-Reign of Legends. But in the original Ruby or Sapphire you can't get it.
No you cannot catch Entei in Pokemon Emerald.